Vandersteen Model 2D?

As far as upgrades, might a carbon tweeter be a good idea? I think it's likely important that Vandersteen keep these below the the 3K price point, so perhaps that is out of the question.

Current retail is $2700. Pretty much zero chance of adding the carbon tweeter and staying south of $3K. RV frequently adds small updates without changing the model name. I bought "Sig IIs" in 2007 but the latest "Sig IIs" have several updates, most notably the tri-woven midrange driver. I don't think Vandersteen can substantially upgrade the model 2 without a correspondingly substantial price increase.
Supposedly, RV hopes to bring pistonic-behaving drivers to a lower price point but this would still be, I think, well above $2700!

elevating pistonic behavior to the same importance as time/phase correct behavior.

Some designers think that pistonic behavior is *more* important, but might as well achieve both if you can!

require a technology different from the current carbon / balsa cones

Yes, RV's carbon technology is unlikely to trickle down to affordable price points. Thiel drivers got pretty close. The CS3.7 midrange was said to have a break up point "well above" 20K cycles. This behavior is especially important with XOs using slow roll-off filters (ie, first order).
I have Thiel CS2.4s. The midrange diaphragms on these are not ribbed like those of the CS3.7. Still, the aluminum is stiff enough to that the break up mode is well suppressed despite the first order filter (chart 3):https://www.soundstagenetwork.com/measurements/thiel_cs24/

non pistonic is out of phase behavior...they are same thing

I don't find the sound of a driver at break up mode the same as a speaker with a high-order filter, if that's what you're meaning. One is distortion and the other is a very subtle sonic signature. I would much rather have something like a Vivid B1 (pistonic but with out-of-phase filtering) than a Vandersteen 3 (phase coherent but not as resolved because of the non-pistonic diaphragms). You can "have it all" with the Vandersteen 7 but that product is just a bit out of my budget :)

speakers a bit closer to price parity

Fair enough. My non-parity example reflects the dearth of both phase coherent designs and designs with pistonic behavior. Both of these groups are very small and the overlap seems to stop at the Vandersteen 7 (maybe 5 Carbon and out-of-production Thiel CS3.7)! But I stand by my point if we compare the Vandy Quatro CT and Vivid B1, which are at the same-ish price point.

the steep slopes make it easier to get pistonic behavior in the passband....

Yes, that’s what I implied earlier ("especially important with XOs using slow roll-off filters "). Even with steep filters you need a well-behaving driver.

I have lived with Thiels or Vandersteens for over 15 years. So, I am very familiar with phase coherent sound. But during that time I’ve listened to many other speakers. My favorites, alphabetically, are Avalon Ascent, TAD Ref 1, Vandersteen 7, and Vivid Giya. Only the 7 is phase coherent, although the Ref 1 has a coincident midrange/tweeter. But one thing these designs have in common is pistonic behavior. IME, that one factor best corresponds to high resolution and transparency.

I wonder if RV could implement a model 2 CT for under 4K?

IIRC, the price increase from Treo to Treo CT was about $1000. The XO probably also would need to be tweaked or further optimized. Is there really a market for a $4K Model 2? Maybe.
It might also be that RV thinks adding the carbon tweeter is reaching a point of diminishing returns for that product. I know, for example, that he thinks the Treo cabinet is a big jump, sonically, from what he can achieve in the 2 or 3. The Sig II is a really good speaker. RV probably figures serious listeners wanting an upgrade will pony up for the Treo CT.
What *I* would like is a Treo CT with a carbon midrange. But that would probably retail for >$15K and that's out of my budget!

If RV ever does put a carbon tweeter in the 2s (I doubt it), it would be a no-brainer to just call it the "2Carbon".